"The longest migration of any known animal is that of the arctic tern, which travels 10,000 miles from the North Pole to the South Pole and back again each year!"

16,000 km(one way)

Terns are among the most common water birds of ocean and coastal
zones. Species live inland on every continent and in all major
bodies of water. The common tern lives in all parts of the Northern
Hemisphere as well as in South America and Africa. Terns are generally
smaller than their counterparts, Gulls. They vary from 23 to 58
centimeters in length (9 to 23 inches), have a slim streamlined
body with a long forked tail, and carry their bills pointed downward.
Terns are white to gray with white underparts. Terns can hover
over the water and plunge for their food, however in most cases
they will not rest or swim in the water. Surprisingly, they are
scavengers and will eat live or dead fish as well as any other
fresh or rotting food found in or near the water. They are sociable
birds and nest in colonies of thousands.

The arctic tern is not easily distinguished from the common
tern except for one small detail. Migration for any tern is usually
a long one. Many fly across the Atlantic, others from northern
to southern continents. Arctic Terns in particular will travel
the farthest distance of any tern and of any known animal. Its
summer home is in the upper parts of the North Atlantic and the
Arctic and the winter home is the Antarctic. Each leg of its annual
journey is approximately 11,000 miles, one way!